Reciprocating linear actuator

ABSTRACT

A reciprocating linear actuator having a housing receiving a tubular member having a lateral opening with a shaft passing through the opening. A plurality of wheels are also mounted in the housing with the wheels being disposed against the shaft for initiating and controlling linear movement of the housing with respect to the shaft that is rotated by the source of power. The housing is made of a special sheet metal construction, and the wheels are urged against the shaft by a single spring in a special novel arrangement. In use, the reciprocating linear actuator can be made a part of a carriage assembly of a therapeutic massaging chair, and can be fitted with shock absorbers so that reversal of movement for the chair can be achieved in a silent manner.

FOREIGN PATENTS OR APPLICATIONS O Umted States Patent {151 3,673,874 Zaruba et al. v July 4, 1972 54] RECIPROCATING LINEAR ACTUATOR 760,443 10/1956 Great Britain .74/25 [72] Inventors: wenzel Zaruba, East Patterson; Jan 710,135 6/1954 Great Britain ..74/25 nal, Upper Saddle River, both of NJ. Primary Examiner Meyer Peru [73] Assignee: Textol Systems, Inc., Carlstadt, NJ. Assistant Examiner- Wesley S. Ratliff, Jr. Filed: J 1970 Attorney-Caesar, Rivise, Bernstein & Cohen 21 Appl. N0.Z 50,620 ABSTRACT A reciprocating linear actuator having a housing receiving a 52 US. Cl ..74/2s tubular member having a lateral Opening with a shaft Passing 511 Int. Cl ..F16h 21/16 through the p A plurality of wheels are also mounted in [58] Field 6: Search ..74/25 the housing with the wheels being disposed against the Shaft for initiating and controlling linear movement of the housing 5 References Cited with respect to the shaft that is rotated by the source of power. The housing is made of a special sheet metal construction, and UNITED STATES PATENTS the wheels are urged against the shaft by a single spring in a 402,674 5/1889 Judson ..74/25 novel arrangement 2,578,026 12/ 1951 Taylor In use, the reciprocating linear actuator can be made a part of 3,475,972 I l/ 1969 Stiebel a carriage assembly of a therapeutic massaging chair, and can 3,394,599 7/1968 Tucker be fitted with shock absorbers so that reversal of movement 3,434,357 3/1969 Roantree for the chair can be achieved in a silent manner.

6 Claims, 12 Drawing Figures P'ATENTEBJUL 4 m2 SHEET 10F 3 NGE INVENTORS WENZEL ZARUBA JAN SEHNAL ATTORNEY PATENTEDJUL 4 m2 SHEET 20? 3 INVENTORS' WENZEL ZARUBA BY JAN SEHNAL Cam/1., M

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i -com ATTORNEY PATENTEDJUL 4 I972 .SHEET' 3 0f 3 FIG.

FIG. I!

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INVENTORS WENZEL ZARUBA JAN SEHNAL 5% com ATTORNEY RECIPROCATING LINEAR ACTUATOR This invention relates to a reciprocating linear actuator, and more particularly, to a linear actuator that can be manufactured with certain savings in cost, but which includes all of the significant properties of linear actuators that have been heretofore developed.

Until recently, linear actuators were provided with a screw drive so that rotational movement would be used to initiate linear movement.

However, the shortcomings of a screw drive were long recognized. First, the ratio of linear displacement to angular displacement could be changed only by securing a screw drive having threads of a different pitch. Second, the thrust of the shaft will remain constant for the rate of rotation of the shaft. Also, the direction of linear movement can be changed only by changing the direction of shaft rotation.

The foregoing difficulties were overcome by devices shown in both Zaruba US. Pat. No. 3,473,393 and Steibel US. Pat. No. 3,475,972.

In the Zaruba patent there was provided a linear actuator which comprised a plurality of wheels in contact with a rotating shaft, with the pitch of the wheels being adjustable and changeable in order to change the rate of movement and direction of linear displacement of the rotating shaft.

In Zaruba co-pending application Ser. No. 851,421, filed Aug. 19, 1969, there is introduced several improvements including a pressure controlled diaphragm by which the pressure of the wheels against the shaft could be varied from a remote source, and thus the thrust of the linear actuator could be varied from a remote source.

With the principle of the modern linear actuator now established, it has become important to develop economies in the device in order to hasten its use in industry.

It is therefore an object of the present invention to provide a reciprocating linear actuator which can be manufactured at a cost lower than similar devices that have already been proposed.

Yet another object of the present invention is to provide a reciprocating linear actuator in which the pitch of the wheels can be varied or the direction of linear actuation reversed while the device is in use.

Still another object of the present invention is to provide a reciprocating linear actuator including a low cost housing and a special tubular member located in the housing including a construction whereby a single spring is relied upon to urge the opposed wheels against the rotating shaft.

A further object of the invention is to include a safety device to prevent erratic reciprocation of the linear actuator in the event the control member fails to function properly.

Still another object is to provide a linear actuator having pairs of opposing wheels to increase thrust and provide a more positive switching action.

The foregoing as well as other objects of the invention are achieved by providing a reciprocating linear actuator which includes a special housing and a tubular holder located within the housing. The wheels of the linear actuator are located within the tubular holder which also includes an opening to pennit the passage of the rotating shaft. The tubular holder also includes a single spring that is internally contained in order to urge both pairs of wheels against the rotating shaft.

The reciprocating linear actuator is adapted to be incorporated into the carriage assembly of a therapeutic massaging chair and is fitted with shock absorbers so that reversal of the carriage assembly is not accompanied with an annoying clicking noise.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is an elevational view, partially in section showing an embodiment of the reciprocating linear actuator of the present invention;

FIG. 2 is a sectional view taken along the lines 2-2 of FIG.

FIG. 3 is a sectional view taken along the lines 3-3 of FIG. 2 and showing the addition of shock absorbers for silencing purposes to cushion the stopping and the control member as it is moved from a forward to a reverse position;

FIG. 4 is a perspective view showing the tubular holder;

FIG. 5 is a sectional top plan view showing a second embodiment of the present invention which utilizes two pairs of wheels;

FIG. 6 is a sectional view taken along the lines 66 of FIG.

FIG. 7 is a perspective view showing the pairs of wheels in actual contact with the rotating shaft;

FIG. 8 is a schematic plan view of the carriage for providing the massaging action by means of rubber balls;

FIG. 9 is a sectional view taken along the lines 9-9 of FIG.

FIG. 10 is a view showing the reciprocating linear actuator of the present invention secured to a carriage assembly of a therapeutic massaging chair;

FIG. 11 is a view showing the carriage assembly as associated with a massaging table; and

FIG. 12 is a view showing the use of two reciprocating linear actuators to change the position of both the head and the foot of a hospital bed.

Referring now in greater detail to the various figures of the drawings wherein similar reference characters refer to like parts, a reciprocating linear actuator 20 constituting a first embodiment of the invention is shown in FIGS. 1 to 4, with the use of such an actuator 20 as a part of a carriage assembly 24 associated with the rear cushion 26 of a therapeutic massaging chair being shown in FIG. 10. In FIG. 11, the assembly 20 is associated with a massaging table and in FIG. 12 two reciprocating linear actuators are associated with the head and foot of a hospital bed.

As can be seen in FIG. 1, the linear actuator 20 is adapted to move along shaft 22, depending upon the inclination of the wheels 42 as will be discussed hereinafter.

The shaft 22 is rotatably mounted at 28 and 30 (FIG. 10) and is rotated by motor 32 which is attached to the shaft 22 by a pulley assembly 34.

With further reference to FIG. 10, it will be seen that as the shaft 22 rotates, the linear actuator moves back and forth along the shaft 22, so long as the wheels 42 are inclined with respect to the shaft 22 (i.e. the planes of the wheels are not normal to the axis of the shaft).

With reference again to FIG. 1, it will be seen that the linear actuator 20 includes a rectangular housing 36, which is formed from a blank. A bronze bushing 38 is located at each end of the housing 36, and it is to be noted that the housing 36 also has openings in each of its end walls 37 to permit passage of the shaft 22.

Extending transversely through appropriate openings 38 (transverse to shaft 22) is a cylindrically shaped tubular member 40 (FIG. 2) which acts to hold wheels 42 and 44.

The wheels 42 and 44 are both bearing wheels which are rotatably mounted on pins 46 and 48.

The details of the tubular member 40 can be seen in FIG. 4, and wherein the pin 46 of the wheel 42 is shown to be actually secured for pivotal motion of the wheel 42 only in the tubular holder 40.

The wheel 44, however, is mounted on pin 48 which is secured on rotatable holding yoke 50. By virtue of the foregoing arrangement, the rotation of the wheel 42 is confined to its pivotal attachment to the holder 40. However, the wheel 44 can rotate in a plane as determined by the position of rotatable yoke 50 with respect to the holder 40. This will become clear by reference to FIG. 4 which shows a first control pin 52 secured to and extending from holder 40 for controlling rotation of the plane of wheel 42 (not shown) and a second control pin 54 secured to and extending from rotatable yoke 50.

The first control pin 52 can only pivot the wheel 42 whereas the second control pin 54 moves within a parallel slot 56 and can pivot only wheel 44.

From the foregoing, it will be seen that the wheels 42 and 44 can have their planes of rotation changed by movement of the control pins 52 and 54.

Each of the pins 52 and 54 have ball ends as seen in FIGS. 1 and 2. As shown in FIG. 3, the ball ends of. both pins are contacted by the parallel edges of openings 58 in a pivotable control member 60 so that the position of one of the wheels cannot be changed without making a corresponding change in the position of the other wheel. In this way, the position of the wheels is always opposing as illustrated in FIG. 7 with respect to the alternate embodiment.

The control member 60 is pivotally secured (FIG. 2) to the bottom plate 62 of the housing 36 by means of a rivet 64. As further seen in FIG. 2, the rivets 64 extends through a bronze bushing 66 that is provided in the opening of the control member 60. A pair of nylon washers 68 are provided between the bottom wall 62 of the housing and the control member 60 and between the control member 60 and a metal washer 70.

It can be seen from FIG. 2 that the control member 60 rotates about an axis which is perpendicular to the axis of shaft 22. It should be further kept in mind that the control member 60 is rotated for the purpose of changing the orientation of the wheels 42 and 44 with respect to the shaft 22.

As will be discussed hereinafter in connection with FIGS. 10, 11 and 12, the control member 60 is relied upon in order to achieve a reversal of movement of the linear actuator 20. When the control member 60 rotates, the pins 52 and 54 act to rotate the wheels 42 and 44 with respect to the axis of the shaft 22. Thus, in a first orientation of the wheels 42 and 44 the linear actuator may be travelling from left to right as viewed in FIG. 10. When the linear actuator reaches the extreme right position, the control member 60 will be moved so that the orientation of the wheels 42 and 44 with respect to the shaft 22 is changed, and thus the linear actuator starts to move from right to left as shown in FIG. 10.

As can be seen in FIG. 2 the wheel 44 is urged against shaft 22 by a plurality of washer Springs that are secured against one side of thrust bearing 74 which is held in place by a threaded end cap 76. The thrust bearing 74 separates washer springs 72 from the yoke 50 so that the yoke 50 can rotate with respect to the washer springs 72.

It should be noted that the amount of pressure applied to the shaft 22 by the wheel 44 will cause an equal amount of pressure to be applied by the wheel 42 against the shaft 22 since the wheel 42 cannot move laterally and can only oppose the force exerted by the washer springs 72. In this way only one of the wheels 42 or 44 need have spring pressure, and at the same time there is good assurance that the same amount of pressure will be applied by wheels 42 and 44 against the shaft 22.

As seen in FIGS. 1 and 3 an elongated rod 78 is slidably mounted in a pair of end flanges 80 which depend vertically from the bottom wall 62 of housing 20. Flanges 80 include bushings 84 in which the rod 78 is adapted to slide back and forth in the direction of arrows 82 of FIG. 3. Referring to FIG. it can be seen that the rod 78 extends beyond the ends of the carriage 24. As the linear actuator approaches one of the end mountings 86, the rod 78 is initially urged in a direction opposite to the movement of the linear actuator 20.

Assuming that the rod 78 is in the position of FIG. 3, the linear actuator 20 would be travelling from left to right (as seen in FIG. 10) towards the upper mounting 86 of the chair cushion 26. As the rod 78 abuts the mounting 86, the rod 78 will be slid from right to left as seen in FIG. 3. As this occurs the spring 90 (FIG. 3) is caused to move over the center of rivet 64. Since the other end of the spring 90 is secured to a boss or projection 92 extending from control member 60, the sliding of the rod 78 causes the control member 60 to pivot about axis 64.

When the spring 90 moves past rivet 64, the control member 60 pivots about an axis through the rivet 64, and this pivoting action continues until the control member 60 contacts shock absorber 94 (FIG. 3). The shock absorber 94 is present so that the pivoting of control member 60 may be terminated in a silent manner, as compared with the annoying click that is present in devices currently on the market.

The rotation of the control member 60 causes the planes of wheels 42 and 44 to be reversed in pitch with respect to the axis of shaft 22, and this in turn reverses the direction of movement of housing 20. Thus, in the example being discussed, the housing 20 will now move from right to left as viewed in FIG. 11 until the rod 78 contacts the lower mounting 86 of the rear cushion 26 of the chair. As seen in FIG. 3, a plurality of cylindrical shock absorbers 94 are provided on the top surface of the carriage assembly 24.

The shock absorbers 94 act to cushion the stopping of control member 60 as it is moved from a forward to a reverse position so that there will not be any annoying clicking sound. As previously discussed this clicking sound is a problem in massaging chairs currently on the market because many users would like to fall asleep on the chair as the chair is being used to massage their back. However, the clicking action that is normally caused by a reversible motor usually prevents a sound sleep.

FIG. 5 shows a modified version of the linear actuator of the present invention which uses a pair of wheels and 102 in each yoke of the tubular holder 40. The yokes are different in order to mount the pair of wheels 100 and 102.

As best seen in FIG. 7 the desirable action of the wheels 100 and 102 on each side of the shaft 22 is the fact that there is a very solid switching action when the wheels are turned past the so-called neutral point when the plane of the wheels is perpendicular to the axis of the shaft. It should be kept in mind that the pressure exerted by the wheels against the shaft is greatest when the planes of the wheels 100 and 102 are perpendicular to the shaft since the outermost edge of the periphery of the wheels 100 and 102 most directly abut the outer surface ofthe shaft, thereby causing the spring pressure to be greatest.

However, as soon as the wheels move on to their preselected pitch or normal position, the wheels 100 and 102 do not grab the shaft 22 at the outermost point of the wheels with respect to the yokes, but rather in a wedging fashion as seen in FIG. 7 between the wheels. Thus, the switching action is very solid.

It will also be seen that the double wheel arrangement aids the frictional engagement between the wheels of the shaft 22. This increases the thrust of the wheels with respect to the shaft 22 that can be obtained with a spring of given pressure. In the embodiment of FIGS 5 to 9 both wheel yokes for wheels 100 and 102 are maintained in place as shown in FIG. 5 with a pin 104 that holds a spring 106 in a compressed state against thrust bearing 108 that in turn abuts the rear surface of yoke l 10.

Furthermore, as can be seen in FIG. 9, a motor assembly is provided on top of the carriage assembly 24, with the motor assembly 120 being a vibrating motor that provides therapeutic action to the carriage as it moves along the rear surface of the rear cushion.

The same linear actuator 20 can also be used on a massaging table as shown in FIG. 11, with the operation of the linear actuator being essentially identical except the massaging action is applied to the top surface of the massaging table.

In FIG. 12 there is shown a pair of linear actuators that can be used for the movement of a hospital bed. The linear actuator 220 is connected to a rod 224 that is pivotally mounted to the supporting surface 226 of the mattress. When the linear actuator 220 is in the position as shown in FIG. 12, the rod 224 (which is pivotally secured to the housing of linear actuator 220) causes the supporting surface 226 to be moved up to the position shown in FIG. 12.

When the linear actuator moves from lefl to right as seen in FIG. 12 until point A is attained, the supporting surface 226 will be essentially horizontal.

Similarly, a second linear actuator 230 is used to raise and lower the portion 220 of the supporting surface of the mattress. When the linear actuator 230 reaches point B the lower portion of the mattress will be in a horizontal position.

It should be noted that the linear actuator housings have a construction similar to the ones shown in FIG. 1 except that the carriage assembly is not carried with the linear actuator since such a connection is not required in FIG. 12.

In order to operate linear actuators 220 and 230, it is necessary only to actuate a motor 250 that rotates shaft 252 which is common to the linear actuators 220 and 230. Thus, the wheels of the linear actuators 220 and 230 are so disposed at pitches with respect to each other that when shaft 252 is rotated, the linear actuators 220 and 230 will travel towards each other from the position shown in FIG. 12 so that the mattress is brought to a horizontal condition. The matress will reach the condition of FIG. 12 when the linear actuators 220 and 230 return to the extreme position as shown in FIG. 12. Thus, by merely turning on motor 250, the bed can be raised and lowered.

Reference is again made to FIG. 3 which shows a safety device in the form of a pair of shafts 200 and 201 that are located at either side of the head 202 of the control member 60. Should the spring 90 break, then the linear actuator would move for an indefinite time in an erratic manner, depending upon the various positions that the control member 60 might assume as motivated by vibrations from the motor 120 or 250. The shafts 200 and 201 prevent such erratic movement. As shown in FIG. 5, both the shaft 200 and the shaft 201 possess at their outer ends a spring 203 that is held between a boss 204 and a retaining ring 205 (see also FIG. 1).

As best seen in FIG. 5, the shafts 200 and 201 possess opposed inner ends 206, either one of which is free to move toward pin 54. So long as the spring 90 is functioning properly, the abutment of elongated rod 78 against mounting 86 will cause a constant and predetermined switching of the movement of the linear actuator as described hereinabove.

Although either the shaft 200 or 201 will also abut the mounting 86, nothing will happen so long as the spring 90 is operative since the desired switching action will be performed by the sliding of the rod 78. However, should the spring 90 become inoperative, then of course the sliding of the rod 78 will no longer cause movement of the control member 60. In such a case, either the shaft 200 or 201 (depending upon the particular extreme position of the linear actuator) will abut against mounting 86. This will cause an inward sliding of the shaft 200 or 201 such that the inner end 206 will contact pin 54 and urge pin 54 together with control member 60 and pin 52 to the neutral position wherein the plane of the wheels is perpendicular to the shaft 22 so that all further movement of the linear actuator ceases.

The halls 121 are rotatably mounted in pairs on four elongated pins 122. The pairs of balls 121 are secured on pins 122 by a pair of brackets 123 which are each comprised of a pair of spring leaves 124 and 125 which include openings for receiving one end of each of pins 122. The body of carriage 24 also includes a pair of openings for reception of the other end of pins 122. A central bracket member 126 which is secured to the body of carriage 24 extends perpendicular to the brackets 123 and is secured thereto by suitable fasteners 127.

The vibrator 120 is also secured to bracket 126. As the vibrator 120 is operated, the vibrations are applied via the rubber balls 121 which are pressed against the rear surface of the chair or table which includes the vibrating carriage. The abutment of the balls against the rear of the vibrating surface of the chair or table also prevents the rotation of the housing of the linear actuator with respect to the shaft 22.

The actual vibrating device is shown schematically in FIGS. 8 and 9 wherein a vibrating motor 120 causes vibrating motion of a series of balls 121 that rub against the back of the seat in order to transmit the vibrating motion to the user.

It is to be noted that the present invention has been described in terms of a rotating shaft 22 and a carriage 24, the

linear movement of which is caused by the action of the wheels 42 and 44. However, it should be kept in mind that the foregoing motions are relative only, such that the present invention would be applicable to other conditions of use, as for instance, where the housing of the linear actuator is stationary and the shaft is driven linearly.

Without further elaboration, the foregoing will so fully illustrate our invention that others may, by applying current or future knowledge, readily adapt the same for use under various conditions of service.

What is claimed as the invention is:

l. A reciprocating linear actuator including a housing having an opening to permit the transverse passage of a rotating shaft, said housing having mounted therein at least two wheels which are urged against said shaft, the planes of said wheels each being rotatable about an axis perpendicular to said shaft by a control member, a control rod extending substantially parallel to said shaft and beyond the housing of said actuator, a spring attached in an offset manner to said rod and also to said control member with said control rod being carried along with said housing whereby when said housing approaches one of its extreme positions of linear movement, said control rod will abut reversing means, said abutment being against the tension of said spring wherein said control member is caused to pivot against said spring in a dead center action until the control member reaches an opposite position wherein the position of said wheels is reversed to cause a reversal of movement of said carriage.

2. The invention of claim 1 wherein a shock absorbing means is provided adjacent said control member for silencing the reversing action.

3. A reciprocating linear actuator system including a rotating shaft, a housing having an opening to permit the passage of said rotating shaft, said housing having mounted therein a plurality of wheels which are urged against said shaft, the planes of said wheels each being rotatable about an axis perpendicular to said shaft by a control member, limit means mounted adjacent said shaft, said limit means being spaced from each other and act to define the limits of movement of said housing along said shaft, said limit means coacting with said control member as said housing approaches one of its extreme positions of linear movement to cause the rotation of said plane of said wheels to a position perpendicular to the axis of said shaft and means connected to said control member to continue rotation of the plane of said wheels to a position wherein the wheels cause a reversal of movement of said housing.

4. A linear actuator comprising a housing formed of a hollow cylindrical tube having a transversely extending opening to permit the transverse passage of a rotating shaft, said tube acting as a holder for a first wheel and as a housing for a holder for a second wheel, said tube being rotatable about its elongated axis which extends transversely to the axis of said shaft, said holder for said second wheel being rotatable in said tube coaxially with respect to said tube, said first wheel being so mounted in said tube that the plane of said wheel is rotatable with said tube and said wheel is urged against said shaft on one side of said tube and said second wheel is so mounted in said holder therefor that the plane of said wheel rotates with respect to said tube and is urged against said shaft on the other side of said tube, said planes of said wheels each being similarly inclined with respect to said shaft, and means for controlling the inclination of said wheels, said means for controlling causing rotation of said tube and said holder for said second wheel in opposite directions as said planes of said wheels are rotated to a different inclination with respect to said shaft.

5. The linear actuator of claim 4 wherein said first and said second wheels each comprise a pair of closely spaced wheels.

6. The linear actuator of claim 4 wherein said first wheel is rotatably mounted on a transverse axis which is stationarily positioned longitudinally of said tube and said second wheel is rotatably mounted in said holder therefor with said holder being longitudinal lyslidable in said tube, and means adjacent to said holder for said second wheel for resiliently urging said second wheel against said shaft, whereby said first wheel has a resultant equal and opposite urging force applied between said first wheel and said shaft.

a r-tw t 

1. A reciprocating linear actuator including a housing having an opening to permit the transverse passage of a rotating shaft, said housing having mounted therein at least two wheels which are urged against said shaft, the planes of said wheels each being rotatable about an axis perpendicular to said shaft by a control member, a control rod extending substantially parallel to said shaft and beyond the housing of said actuator, a spring attached in an offset manner to said rod and also to said control member with said control rod being carried along with said housing whereby when said housing approaches one of its extreme positions of linear movement, said control rod will abut reversing means, said abutment being against the tension of said spring wherein said control member is caused to pivot against said spring in a dead center action until the control member reaches an opposite position wherein the position of said wheels is reversed to cause a reversal of movement of said carriage.
 2. The invention of claim 1 wherein a shock absorbing means is provided adjacent said control member for silencing the reversing action.
 3. A reciprocating linear actuator system including a rotating shaft, a housing having an opening to permit the passage of said rotating shaft, said housing having mounted therein a plurality of wheels which are urged against said shaft, the planes of said wheels each being rotatable about an axis perpendicular to said shaft by a control member, limit means mounted adjacent said shaft, said limit means being spaced from each other and act to define the limits of movement of said housing along said shaft, said limit means coacting with said control member as said housing approaches one of its extreme positions of linear movement to cause the rotation of said plane of said wheels to a position perpendicular to the axis of said shaft and means connected to said control member to continue rotation of the plane of said wheels to a position wherein the wheels cause a reversal of movement of said housing.
 4. A linear actuator comprising a housing formed of a hollow cylindrical tube having a transversely extending opening to permit the transverse passage of a rotating shaft, said tube acting as a holder for a first wheel and as a housing for a holder for a second wheel, said tube being rotatable about its elongated axis which extends transversely to the axis of said shaft, said holder for said second wheel being rotatable in said tube coaxially with respect to said tube, said first wheel being so mounted in said tube that the plane of said wheel is rotatable with said tube and said wheel is urged against said shaft on one side of said tube and said second wheel is so mounted in said holder therefor that the plane of said wheel rotates with respect to said tube and is urged against said shaft on the other side of said tube, said planes of said wheels each being similarly inclined with respect to said shaft, and means for controlling the inclination of said wheels, said means for controlling causing rotation of said tube and said holder for said second wheel in opposite directions as said planes of said wheels are rotated to a different inclination with respect to said shaft.
 5. The linear actuator of claim 4 wherein said first and said second wheels each comprise a pair of closely spaced wheels.
 6. The linear actuator of claim 4 wherein said first wheel is rotatably mounted on a transverse axis which is stationarily positioned longitudinally of said tube and said second wheel is rotatably mounted in said holder therefor with said holder being longitudinally slidable in said tube, and means adjacent to said holder for said second wheel for resiliently urging said second wheel against said shaft, whereby said first wheel has a resultant equal and opposite urging force applied between said first wheel and said shaft. 